This section is intended to introduce the reader to various aspects of art that may be related to aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
High-intensity discharge (HID) lamps are often formed from a ceramic tubular body or arc tube that is sealed to one or more end structures. The end structures are often sealed to this ceramic tubular body using a single seal glass. Sealing usually involves heating the assembly of the ceramic tubular body, the end structures, and the seal glass to induce melting of the seal glass and reaction with the ceramic arc tube and the end structures to form a strong chemical and physical/mechanical bond. The ceramic tubular body and the end structures are often made of the same material, such as polycrystalline alumina (PCA). Thus, the single seal glass may have physical and mechanical properties matching those of all of the ceramic components, i.e., the ceramic arc tube and the ceramic end structures.
However, certain applications may require the use of different materials for the ceramic arc tube and the end structures. Unfortunately, various stresses may arise from the sealing process, the interface between the joined components, and the materials used for the different components. For example, the materials of the ceramic arc tube, the end structures, and the single seal glass may have different mechanical and physical properties. These properties generally include different coefficients of thermal expansion (CTE), which can lead to residual stresses and sealing cracks. These potential stresses and sealing cracks are particularly problematic for high-pressure lamps and operational conditions involving rapid cycling.
The geometry of the interface between the ceramic arc tube and the end structures also may attribute to the foregoing stresses. For example, the end structures are often shaped as a plug or a pocket, which interfaces both the flat and cylindrical surfaces of the ceramic arc tube. If the components have different coefficients of thermal expansion and elastic properties, then residual stresses arise because of the different strains that prevent relaxation of the materials to stress-free states. For example, in the case of the plug type end structure, if the plug has a lower coefficient of thermal expansion than the ceramic tubular body and seal glass, then compressive stresses arise in the plug region while tensile stresses arise in the ceramic arc tube.
In addition to the ceramic arc tube and end structures, high-intensity discharge lamps also include a variety of internal materials (e.g., gases) and electrode materials to create the desired high-intensity discharge for lighting. The particular internal materials disposed in the high-intensity discharge lamps can affect the sealing characteristics, the light characteristics, and the type of materials that may be workable for the lamp components and the seal glass. For example, certain internal materials, such as halides and metal halides, may be desirable for lighting characteristics, but they are corrosive to some of the ceramic and metallic components that comprise the tubular body and end structure.
Accordingly, a technique is needed to provide a lighting system, such as high-intensity discharge lamp, with improved sealing characteristics.